Molecules in a Hurry to Get Rid of Antiaromaticity

When light strikes an aromatic molecule, the electrons rearrange, and the compound can gain antiaromatic character, becoming especially reactive. From there, the drive to escape excited-state antiaromaticity can trigger all sort of photochemical reactions—the clock is ticking, and the molecule is in a hurry to get rid of antiaromaticity. For example, benzene is [4n+2] π-aromatic in the ground state, but [4n+2] π-antiaromatic in the excited state. To alleviate excited-state antiaromaticity, benzene quickly isomerizes to fulvene or even to the highly strained benzvalene. This dissertation focuses on demonstrating the important consequences of excited-state antiaromaticity in photoinduced electron and proton transfer reactions. Upon photoexcitation, o-salicylic acid converts to the “rare” keto tautomer by proton transfer, and this alleviates excited-state antiaromaticity of the π-ring. In the photoinduced electron transfer reaction of phenol, leading to O–H bond cleavage, a π-electron departs from the photoexcited π-ring and relieves antiaromaticity. The photoinduced proton-coupled electron transfer reaction of Watson–Crick DNA base pairs is another example of excited-state antiaromaticity relief. Transfer of an electron and a proton from the photoexcited purine to the pyrimidine significantly alters the π-system of the paired bases. The computational findings presented here provide valuable insights for understanding the photoreactions of many “aromatic compounds” of chemical and biological relevance.